The River Danube flows through four national capitals: Vienna, Bratislava, Budapest and where?

The Danube flows southeast through four capital cities, Vienna, Bratislava, Budapest, and Belgrade. It is 2,860 km (1,777 mi) long and also flows through or makes part of the border of ten European countries: Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Romania, Bulgaria, Moldova and Ukraine. The river originates in the town of Donaueschingen, in the Black Forest of Germany, where the rivers Brigach and Breg form a single channel and flows until it reaches the Black Sea.

Belgrade is the capital city of the country of Serbia. The River Sava joins the River Danube in the city. The historical part of Belgrade lies on the right bank of both rivers and "New Belgrade" lies on the other side. The modern or "new" part of the city gradually developed after the city expanded following the Second World War.

The Danube played a vital role in the settlement and political evolution of central and southeastern Europe. As a result, its banks are lined with castles and fortresses, and once formed the boundary between large empires. Its fame has led to several pieces of music named in its honour, most notably the waltz 'An der schönen, blauen Donau' (1867; 'The Blue Danube') by Johann Strauss the Younger.